1. Field of Invention
The invention relates to a planar transformer with a core, a first printed circuit board and a second printed circuit board, wherein the first printed circuit board has a first winding and the second printed circuit board has a second winding and the planar transformer is suitable for intrinsically safe electronic circuits.
2. Description of Related Art
Regulations for the construction and testing of intrinsically safe operating means, which are intended for use in areas in danger of gas explosions, as well as for related operating means, which are intended for the connection to intrinsically safe electronic circuits that lead into such areas, are specified by the standard EN 60079-11. This standard is also valid for electric operating means or parts of electric operating means, which are found outside of the area in danger of gas explosions or are protected by another type of ignition protection according to IEC 60079-0, insofar as the intrinsic safety of the of the electronic circuits can be dependent on the construction and design of this electric operating means or on parts of this operating means in the area in danger of gas explosions.
The type of protection “intrinsic safety” is based on the limitation of electric energy within operating means and connecting cables, which are subjected to an explosive atmosphere at a level below which ignition can be caused either by sparking or heating.
An essential requirement, which results from this standard, is the compliance of separation gaps between conductive parts of intrinsically safe and non-intrinsically safe electronic circuits, different intrinsically safe electronic circuits and an intrinsically safe electronic circuit and grounded or insulated metallic parts. The separation gap is understood here as the shortest gap between two conductive parts. Furthermore, a distinction results according to the medium, through which the path of the shortest gap leads, and correspondingly, different separation gaps result for air gaps, gaps in grouting, gaps through firm insulation, creepage distances in air and creepage distances under a protective layer. Subsequently, the compliance of these separation gaps depends on the maximum occurring voltage between the conductive parts.
A stipulated separation gap can also be composed of multiple sections added together. Sections which are smaller than one third of the respective separation gap for this medium are seen as non-countable short circuit failures, insofar as the intrinsic safety is compromised. Sections, which are greater than or equal to one third of the respective separation gap, are seen as countable short circuit failures, insofar as the intrinsic safety is thereby compromised.
The importance of the distinction in countable and non-countable short circuit failures lies in up to two countable short circuit failures being used in the evaluation of intrinsic safety, plus the non-countable short circuit failures, which yield the most unfavorable conditions.
Furthermore, the sections can also run through another medium. Here, an additive composition occurs at an equivalent separation gap by a hypothetical single medium using fixed weighting factors. Observance may find only sections here that do not lie below one third of the corresponding separation gap.
U.S. Pat. No. 5,321,380 describes a planar transformer having a first printed circuit board and at least one planar magnetic element thereupon. The planar magnetic element includes a first transformer winding and a magnet core, which extends through an opening of the first printed circuit board and is surrounded by a first transformer winding. A second transformer winding is arranged on a second printed circuit board, which also has an opening through which the magnet core extends.
A planar, intrinsically safe transformer is known from European Patent Application EP 0 715 322 B1, in which every layer of a multilayer board has a transformer winding on at least one side and at least one of the layers of the multilayer board has a transformer winding on both sides; an additional insulation layer being arranged between every pair of neighboring transformer windings. At least one part of each transformer winding is surrounded here by a magnet core. In particular, each of the first layers, which have a transformer winding on each side, has the same thickness.
Due to the normative specifications regarding separation gaps to be kept, the printed circuit boards used in the prior art have a layer thickness which directly corresponds to the specified separation gaps. The printed circuit boards required for this, having exceptionally large layer thicknesses, however, are excessively costly and result in large construction heights of the transformers.